1) Field of the Invention
The present invention relates to a tension reducer, and particularly to a tension reducer suitable for use in a webbing retractor.
2) Description of the Related Art
This type of tension reducer (webbing retractor) has two types of spiral springs: one having a strong resilient characteristic and the other having a weak resilient characteristic (large and small). One end of the weak resilient spiral spring is connected to a webbing take-up spindle, whereas the other end thereof is connected to the strong resilient spiral spring by way of a gear wheel. The other end of the strong resilient spiral spring is fastened to a case (cover). That is, the take-up spindle, the weak resilient spiral spring, the gear wheel and the strong resilient spiral spring are connected in series to one another. The take-up spindle is urged in a webbing winding direction by these spiral springs. In addition, a pawl lever is provided in the vicinity of the gear wheel and can be brought into engagement with the gear wheel to block its rotation. The operation of the pawl lever is interlocked with the activation of a solenoid or the opening/closing of vehicle doors. Further, the weak resilient spiral spring is always kept in a fully close-wound state before the webbing is fastened to an occupant.
The strong resilient spiral spring is wound tight in accordance with the rotation of the take-up spindle while the occupant is pulling out the webbing therefrom to put it on. The weak resilient spiral spring connected in series to the strong resilient spiral spring is kept in the fully close-wound state over a period in which the strong resilient spiral spring is being wound tight. The pawl lever is activated by an operating mechanism immediately after the occupant puts on the webbing so as to cause the gear wheel to stop rotating. That is, the tension reducer is in an activated state. Thus, the urging force of the wound-tight, strong resilient spiral spring in the webbing winding direction is not exerted on the take-up spindle, and hence the webbing is pulled out only by the weak resilient spiral spring. As a consequence, the occupant feels unrestricted by the pressure of the webbing.
In general, the torque characteristics of the weak resilient spiral spring are graphically represented by hysteretic and non-linear curves as shown in FIGS. 4 and 8. When the occupant attempts to put on the webbing, the weak resilient spiral spring is in a fully close-wound state before the webbing is pulled out of the take-up spindle. Therefore, the torque of the weak resilient spiral spring represents the maximum value at the point H shown in each of FIG. 4 and 8. Since the weak resilient spiral spring is kept in the fully close-wound state even while the webbing is being pulled out by the occupant, the torque of the weak resilient spiral spring represents the value at the point H. The state referred to above continues until the tension reducer starts to operate. When the webbing is fastened to the occupant, the tension reducer is activated. As a consequence, the urging force of the strong resilient spiral spring is not exerted on the take-up spindle and only the urging force of the weak resilient spiral spring is exerted on the take-up spindle. However, the webbing is allowed to extend to an increased length at this time in order for the occupant to operate a buckle device. Therefore, the weak resilient spiral spring still remains in the fully close-wound state. Thus, even at this time, the weak resilient spiral spring represents the value at the point H. After the buckle device has been operated by the occupant, the webbing is slightly wound up by the urging force of the weak resilient spiral spring in such a manner that the webbing is exactly fitted to an occupant's body. Therefore, the weak resilient spiral spring falls into a slightly-loosened state. Thus, the torque of the weak resilient spiral spring at this time is represented as a slightly-lowered value, i.e., a value at the point I shown in each of FIGS. 4 and 8.
On the other hand, when the occupant desires to release the fastened webbing, the blocking of the gear wheel by the pawl lever is released so that it may rotate. Namely, the tension reducer is put into a released state. Therefore, the urging force produced by the strong resilient spiral spring in the webbing winding direction is exerted on the take-up spindle again, so that the webbing is rapidly wound up thereon by the strong resilient spiral spring. Since the weak resilient spiral spring connected in series to the strong resilient spiral spring has an urging force smaller than that of the strong resilient spiral spring, it is wound tight by the urging force of the strong resilient spiral spring. When the webbing is fully wound up on the take-up spindle, the rotation of the take-up spindle is stopped. However, in this case, the weak resilient spiral spring has been kept in the fully close-wound state.
The torque of the weak resilient spiral spring at the time that the tension reducer is deactivated and the weak resilient spiral spring is wound tight by the urging force of the strong resilient spiral spring is represented by a value at a point J shown in each of FIGS. 4 and 8. The change in torque developed when the weak resilient spiral spring is wound tight by the urging force of the strong resilient spiral spring is represented by a curve indicative of a transition from the point J to the point H. The torque of the weak fully wound up on the take-up spindle is represented by the maximum value at the point H because the weak resilient spiral spring is in the fully close-wound state.
Thus, the region indicative of the degree of tightening of the windings of the weak resilient spiral spring in the conventional tension reducer corresponds to the range represented by G in each of FIGS. 4 and 8. Therefore, the range of the torque to be used is from the value at the point H to the value at the point I in the same drawing. When the torque region to be used is compared with an entire torque curve, it is found that the weak resilient spiral spring is used in the range in which the spring torque is considerably high. In addition, the difference in the spring torque between the points I and H is represented by .DELTA.T, and the weak resilient spiral spring is employed in a region where the change in torque is great.
However, when the urging force of the weak resilient spiral spring becomes large after the occupant puts on the webbing and the tension reducer is activated, the webbing is pulled out excessively, so that the feeling of applied pressure to whoever wears the webbing is recognized.
When it is desired to adjust the length of the webbing according to movements of the body or the like after the occupant has put on the webbing and the tension reducer is activated, the pulling out of the webbing against the urging force of the weak resilient spiral spring or the winding up of the webbing by the urging force of the weak resilient spiral spring is performed. When the urging force of the weak resilient spiral spring is not constant at this time, there is a situation in which the pulling out or winding up of the webbing cannot be accomplished as the occupant intends.